The interferons (IFNs) are proteins which inhibit, specifically, the growth of viruses in cells. The `antiviral` effect of the IFNs is exhibited by minute (10.sup.-13 M) IFN concentration and therefore most assays for IFN are based on the quantitation of this effect.
Since the direct quantitation of virus yield by the `plaque assay` is too laborious to be applied as an assay for IFN, a variety of alternative techniques has been developed. In most cases the virus yield is estimated indirectly by determining the extent of cell destruction by the virus.
This technique has several severe limitations: not in all types of cells is the viral cytopathic effect clear enough to be easily estimated and many other agents, besides viruses, can increase cell death and can, therefore, interfere with this `cytopathic effect` assay of IFN.
Other techniques which are used for estimation of the antiviral effect are based on quantitation of various other viral functions including certain viral enzymatic activities and hemagglutination activity of viruses. These techniques can be used only for those viruses which exhibit the measured activity and are therefore applicable only with limited cell-virus combinations.
A more general technique which makes possible the direct quantitation of viral components, in an accurate and sensitive manner, is, therefore, necessary for a reliable determination of IFN.